The invention involves an optical near-field probe with an optical (light) waveguide and a tip manufactured by micro-engineering that is attached to a carrier component. The invention also includes a process for manufacture of an optical near-field probe.
Optical near-field probes are used in scanning near-field optical microscopy (SNOM), as is described for example in "Near-field Optics: Light for the World of NANO" in J. Vac.Sci. Technol., B (12) 3, pages 1441-1446 (1994).
Optical near-field microscopy is based on the scanning of a surface using an optical aperture in order to reach resolutions better than the Abbe-limit. From App. Optics, vol. 34, no. 7, pages 1215-1228 it is known, for example, to use pointed glass fiber ends, which obtain a small aperture by vapor deposition of a metal layer. Optical signal detection alone has the disadvantage, however, that no separation of topographical and optical effects is possible. For the separation of these two effects it is necessary to adjust the glass fiber tip during scanning to a constant distance of several nanometers from the sample surface, which is possible only at a considerable adjustment expense in known near-field probes because of their method of construction. Furthermore, there is the risk without distance control that during scanning the glass fiber tip or the sample surface will become damaged.
In order to improve both of these aspects, shear force detection was developed, which is described in U.S. Pat. No. 5,254,854, for example. Shear force detection functions as a distance control, in order to adjust the distance between fiber tip and sample surface to a constant value. To do this, the glass fiber end is generally shifted by forced oscillations of a piezo ceramic element in constant oscillations parallel to the sample surface. The oscillation of the glass fiber end is damped in proximity to the sample surface, such that this damping becomes larger the closer the glass fiber tip is to the sample surface. In order to measure the damping of the oscillation, the glass fiber end is generally illuminated using a laser diode mounted orthogonally to the plane of oscillation, such that the changing shadow of the glass fiber modulates the intensity on a photodetector. This intensity modulation corresponds to the mechanical oscillation amplitude and functions electrically rectified as an input signal for a control circuit. Using a signal of the control circuit, another piezo ceramic element is controlled which shifts the distance of the sample surface to the glass fiber end to such an extent that an externally prescribed target value is obtained. In this way, the sample surface can be scanned by the tip, and the distance between tip and sample can always be followed or adjusted to a constant value.
For this it is necessary, among other things, that the near-field probe have a suitably large angular freedom on the one hand, i.e. that it can be brought to the sample without a large adjustment expense, and on the other hand, that it be constructed to be as thin as possible because the material thickness determines the damping and thus the resolution. Furthermore, it is desirable to construct the near-field probe in a way such that it can be manufactured as a mass-produced product.
It is known from U.S. Pat. No. 5,294,790 to manufacture the optical near-field probe from two components, namely glass fibers and tips. The disadvantage of the device described there consists in that the glass fiber is arranged at a distance to the membrane and thus to the near-field tip, so that an intermediate space is present which entails light losses that can only be avoided if the intermediate space is filled with a lens or an immersion liquid. Furthermore, the mounting of the near-field tip of this type is so voluminous that it cannot be used for shear force detection. In particular, it is not possible without a considerable adjustment expense to approach the tip to the sample surface, because a probe slope of only .+-.1.degree., relative to the normal line to the sample plane is allowed. With a larger probe slope edge areas of the probe would sit on the sample surface. Furthermore, the oscillation behavior for shear force detection is poor because of the large mass of the mounting structure on the fiber end, i.e., the probe has a small oscillation quality.
From U.S. Pat. No. 5,166,520 a near-field probe is known which is not, however, suitable for optical application purposes, but instead is used specially for conductivity measurements. To manufacture probes suitable for this, a glass pipette is taken as a start, on the end of which a membrane with a hollow tip is mounted. In the hollow tip an electrolyte is introduced for example, which is connected to a detection device via a suitable electric connection. This near-field probe is set directly onto the sample surface, so that the conductive material in the tip of the probe contacts the sample. In order to prevent damage, the membrane must be mounted in such a way that it is also flexible after being attached to the pipette and can be oscillated. For the membrane and the tip non-transparent materials are used. Near-field probes of this type cannot be used either for scanning near-field optical microscopy or for shear force detection, because on the one hand, the pipette is too rigid for this, and on the other hand, the membrane is fixed in its oscillation. Damage to the membrane is easily possible.